Two of the most widely used processes for removal of H.sub.2 S from process gas streams are the redox processes that use 1) an iron chelate absorption solution, e.g. LO-CAT.RTM., and 2) a vanadium-based redox absorption solution, e.g. the Stretford process. The oxidation-reduction reactions that permit these processes to be carried out continuously are well known to those skilled in the H.sub.2 S removal art and are well documented in the literature. The ferric iron chelate-H.sub.2 S reactions can be represented as follows: EQU H.sub.2 S(gas)+H.sub.2 O(Liquid).revreaction.H.sub.2 S(aqueous)+H.sub.2 O(Liquid) EQU H.sub.2 S(aqueous).revreaction.H.sup.+ +HS.sup.- EQU HS.sup.- .revreaction.H.sup.+ +S.sup.= EQU S.sup.= +2(Fe.Chelate).sup.+3 .fwdarw.S.degree. (solid)+2(Fe.Chelate).sup.+2
By combining these equations, the resulting equation is: EQU H.sub.2 S(gas)+2(Fe.Chelate).sup.+3 .fwdarw.2H.sup.+ +2(Fe.Chelate).sup.+2 +S.degree.
In order to have an economical, workable process to effect catalytic oxidation of the hydrogen sulfide using iron as the polyvalent metal, it is essential that the hydrogen sulfide gas be brought continuously into intimate contact with an iron chelate redox solution and that the ferrous iron chelate formed in the above described manner be continuously regenerated by oxidizing to ferric iron chelate by intimate contact with dissolved oxygen, preferably from ambient air. The series of reactions that take place when regenerating the required ferric iron chelate can be represented by the following equations: EQU O.sub.2 (gas)+2H.sub.2 O.revreaction.O.sub.2 (aqueous)+2H.sub.2 O EQU O.sub.2 (aqueous)+2H.sub.2 O+4(Fe.Chelate).sup.+2 .fwdarw.4(OH.sup.-)+4(Fe.Chelate).sup.+3
By combining these equations, the resulting equation is: EQU 1/2O.sub.2 (gas)+H.sub.2 O+2(Fe.Chelate).sup.+2 .fwdarw.2(OH).sup.- +2(Fe.Chelate).sup.+3
The economics and workability of the Stretford process have depended upon a large volume of the vanadium solution, and that the reduced metal vanadate, after absorption of the H.sub.2 S (as HS.sup.- and S.sup.=) to form the metal vanadate in the +4 valance state is continuously regenerated to the +5 valance state by contact with dissolved oxygen for further use of the oxidized metal vanadate in an absorption zone of the process to remove additional H.sub.2 S as elemental sulfur, by which hydrogen sulfide gas is converted to elemental sulfur and water. The Stretford process chemistry is typically summarized according to the following steps:
Absorption and dissociation of H.sub.2 S into alkali: EQU 2H.sub.2 S(g)+2Na.sub.2 CO.sub.3 .fwdarw.2NaHS+2NaHCO.sub.3 ;
Bisulfide oxidation with metavanadate to form elemental sulfur and reduced vanadium: EQU 2NaHS+4NaVO.sub.3 +H.sub.2 O.fwdarw.Na.sub.2 V.sub.4 O.sub.9 +4NaOH+2S; and
Vanadium reoxidation by dissolved molecular oxygen in the presence of ADA: ##STR1##
It is evident from the foregoing equations that theoretically two moles of ferric iron chelate or V.sup.+5 redox solution must be supplied to the absorption zone in which the hydrogen sulfide gas is oxidized to form elemental sulfur for each mole of hydrogen sulfide gas treated, and in actual practice considerably more than the theoretical amount of ferric iron chelate or V.sup.+5 redox absorption solutions are used. In a continuous process of removing hydrogen sulfide by contact with a catalytic ferric iron chelate solution or a metal vanadate (V.sup.+5) solution, catalytic solution is circulated continuously between an absorber zone, where the H.sub.2 S is absorbed by the catalytic ferric iron or metal vanadate (V.sup.+5) solution, and the polyvalent metal redox solution is reduced to ferrous iron or V.sup.+4 in an oxidizer zone where the ferrous iron chelate solution or reduced V.sup.+4 solution is oxidized back to the ferric iron or V.sup.+5 state. Accordingly, either high concentrations of chelated iron or metal vanadate absorption solution is employed in the catalytic solution, or the rate of circulation has been high to assure that sufficient catalytic metal is present for substantially complete absorption of the H.sub.2 S (as HS.sup.- or S.sup.=).
A great many prior art patents are directed to the removal of H.sub.2 S using catalytic polyvalent metal redox solutions, such as an iron chelate or a metal vanadate. Examples of the prior art patents directed to the use of polyvalent metal chelate solution for H.sub.2 S removal include the following U.S. patents: Hartley, et al. U.S. Pat. No. 3,068,065; Sibent, et al. U.S. Pat. No. 3,897,219; Salemme U.S. Pat. No. 3,933,993; Meuly U.S. Pat. No. 4,009,251; Mancini; et al. U.S. Pat. No. 4,011,304; Thompson U.S. Pat. No. 4,189,462; Hardison U.S. Pat. No. 4,238,462; Blytas, et al. U.S. Pat. No. 4,356,155; Hardison U.S. Pat. No. 4,482,524; McManus, et al. U.S. Pat. No. 4,622,212; Primach, et al. U.S. Pat. No. 4,455,287; Fong, et al. U.S. Pat. No. 4,664,902 and U.S. Pat. No. 4,705,676.
One of the most significant problems in the removal of H.sub.2 S gas using a polyvalent metal redox absorption chelate solution, particularly either an iron chelate redox absorption solution or a vanadium-based redox absorption solution, is that the efficiency of the redox reactions required of polyvalent metal chelate solutions is somewhat pH dependent. It is well known that polyvalent metal redox solutions are capable of solubilizing the contaminant metal ions at a pH well above pH 7, but the speed of the redox reactions decreases substantially with decreasing pH, despite statements in issued patents to the effect that a broad range of pH is acceptable--e.g. see Hartley U.S. Pat. No. 3,068,065; Pitts, Jr., et al. U.S. Pat. No. 3,097,925; Meuly, et al. U.S. Pat. No. 3,226,320; Roberts, et al. U.S. Pat. No. 3,622,273. Others have recognized that periodic addition of alkali is needed to maintain a suitably high pH for redox reaction efficiency--e.g. see Roberts et al. U.S. Pat. No. 3,622,273, since the pH tends to drop as the reactions proceed.
As described in the Meuly U.S. Pat. No. 4,009,251, it is recognized that the pH of polyvalent metal redox solutions is lowered during the H.sub.2 S removal (absorption) reactions because of other side reactions between the chelate solution and the H.sub.2 S and the resulting formation of salts formed by polyvalent metal reactions with contaminants dissolved in the polyvalent metal solution after significant contact with the process gas. When H.sub.2 S is the only significant contaminant in the process gas, these side reaction products, as recognized in the Meuly U.S. Pat. No. 4,009,251, are for the most part oxides of sulfur represented by the formula S.sub.x O.sub.y, where x is generally 1 or 2 and y is generally 3 or 4, that are present in an alkali-containing polyvalent metal redox solution as sulfites, sulfates and thiosulfates. If additional reactive contaminant ions are dissolved into an alkali-containing polyvalent metal redox solution from the process gas, for example, if HCN is a significant component in the process gas, thereby providing cyanide ions, CN.sup.-, in the polyvalent metal redox solution, a great many additional side reaction product salts are formed in the polyvalent metal redox solution, particularly side reaction products such as complexes between the polyvalent metal of the redox solution and the cyanide ions; and side reaction products between sulfur compounds (such as thiosulfate, polysulfides and elemental sulfur) sulfide ions and cyanide ions to form thiocyanates and complex metal cyanide complexes.
The more salts that are formed in the polyvalent metal redox solution as a result of a relatively high pH, e.g. above 7, and particularly between about 8 and 9.5, the more frequently it is necessary to add alkali periodically to maintain the desired relatively high pH. As a result, more salts are formed in the polyvalent redox solution, thereby requiring a periodic "blowdown" of polyvalent metal redox solution (a term used to denote the irretrievable discarding of some or all of the polyvalent metal redox solution and replacement with fresh, non salt-contaminated solution). Since the polyvalent metal redox solutions are relatively expensive, the efficiency of the redox reactions catalyzed by polyvalent metal redox solutions at a relatively high pH must be balanced by the expense of the addition of alkali and the expense of lost solution because of "blowdown" being necessary periodically to maintain the redox solution salt concentration below an acceptable level. Further, the sulfur salts formed necessarily reduce the elemental sulfur yield from the H.sub.2 S removal process.
One method disclosed useful to substantially inhibit salt formation in a polyvalent metal redox solution in a process for the catalytic removal of H.sub.2 S from a process gas is disclosed in the Meuly U.S. Pat. No. 4,009,251, using particular polyvalent metal chelating agents to inhibit oxidation of sulfur beyond elemental sulfur.
Some of the most troublesome salts formed in the absorption chamber of a polyvalent metal redox absorption solution process for selective removal of both H.sub.2 S and HCN impurities from a process gas that heretofore have dictated that the HCN be removed from the gas before polyvalent metal redox absorption of H.sub.2 S are, for example, thiocyanates, and various complexes between the polyvalent metal of the redox solution and cyanide ions, e.g. [Fe(CN).sub.6 ].sup.-4 and [Fe(CN).sub.6 ].sup.-3 which remain in the solution as contaminants. The hydrothermal decomposition of thiosulfates and sulfur-cyanide salts, e.g. SCN.sup.-, and the polyvalent metal-cyanide complexes, particularly the polyvalent metal-cyanide complexes [Fe(CN).sub.6 ].sup.-4 and [Fe(CN).sub.6 ].sup.-3, permits the process of the present invention to treat a process gas containing both H.sub.2 S and HCN. This hydrothermal decomposition is made possible by pre-treating the polyvalent metal redox solution in the hydrothermal polyvalent metal precipitation process of the present invention to precipitate the metal as, for example, a polyvalent metal sulfide from the polyvalent metal redox solution, prior to high temperature salt and complex decomposition, to prevent the metal sulfides from fouling the hydrothermal reactor and redox solution preheating devices, e.g. heat exchangers.